In the past, there have been air conditioning systems which have a heat pump section for performing indoor air-warming by using a vapor-compression refrigeration cycle, and a separate heat source section for performing indoor air-warming by using a gas furnace which is a heat source separate from the heat pump section. An example of such an air conditioning system is one in which an air-warming operation by the heat pump section (referred to below as the “heat pump air-warming operation”) is switched to an air-warming operation by the separate heat source section (referred to below as the “separate heat source air-warming operation”) as an outside air temperature drops, and the separate heat source air-warming operation is switched to the heat pump air-warming operation as the outside air temperature rises, as is indicated in Japanese Laid-open Patent Application No. 64-54160.